VIII REPARACIONES
G. Indemnizaciones pecuniarias
AERA is a new antenna system designed to measure short radio pulses emitted by the highest energy cosmic ray air showers. It consists of an array of dozens of antennas sensitive to a frequency range from 30 to 80 MHz with signal processing and electronics developed specifically for this purpose.
The AERA project was carried out in three phases [102]. AERA24 was deployed in September 2010 and consisted of an array of 24 stations equipped with Logarithmic-Periodic Dipole Antenna (LPDA) antennas [168] on a grid with 150 m antenna separation. The LPDA antennas are oriented in magnetic North-South and East-West directions. The signals are amplified and filtered before they are intro- duced into a filter-amplifier and digitization chain. There they are bandpass filtered between 30-80 MHz, digitized and further processed by a Field Programmable Gate Array (FPGA) and a Central Processing Unit (CPU). Later, 100 stations using but- terfly antennas [169] were deployed in May 2013 featuring the second phase of the AERA project, AERA124. The 100 new radio stations in AERA124 are spaced 250 m or 375 m apart from each other. The butterfly antenna is highly sensitive towards the ground which enhances the antenna gain. The general layout of the digitizing chain is similar to the one used in AERA24. Additional antenna prototypes were installed with the goal of measuring the three-dimensional electric field vector. In the third stage, 25 more antenna stations were deployed on a grid with up to 750 m antenna separation in Spring 2015, allowing improved studies of horizontal air showers (zenith angle higher than 60o) with large-scale radio footprints. This last stage features AERA153 which covers an area of about 17 km2. In total, 89 butter- fly antennas are equipped with deeply buffering hardware and 40 butterfly anten- nas are employing internal triggering only. This internal trigger is based on radio self-triggering and small scintillation counters in the electronics compartment of the radio station itself. Due to the increased size of the array, wireless links were intro- duced for the new stations. All 153 stations operate autonomously employing solar power systems. Figure 4.13 shows the LPDA and butterfly station designs deployed at AERA site.
FIGURE4.13: Photos of the radio station LPDA (left) and butterfly (right) de- sign [115].
AERA regularly detects cosmic rays in coincidence with surface and flu- orescence detectors, allowing the cross-calibration of its measurements with the es- tablished baseline detectors of the Observatory. Initial analyses have already led to significant first results, in particular confirming the theoretical predictions of the emission mechanism of radiation in the radio frequency.
It is worth saying that AERA is sensitive to the three main properties of a cosmic ray which are the arrival direction, energy and particle type [102]. The primary energy can be reconstructed from the radiation energy emitted by the elec- tromagnetic shower component in the radio frequency. Therefore, the electric field traces are converted into the energy fluence, i. e., the energy deposit per area, of the electromagnetic wave via the Poynting vector. The measured energy density at all positions of the signal stations is fitted with a two-dimensional lateral distribution function [97,102].
The radiation energy is subject of ongoing work as it can be predicted by Monte Carlo simulations without being strongly influenced by the uncertainties of the hadronic interaction models [170]. Therefore the energy scale of cosmic rays can be studied based on classical electrodynamics. Since the radio emission contains information about the shower development, in particular, the shower maximum, the Xmaxcan be reconstructed from radio emission [171] using the shape parameters of
the hyperbolic radio wavefront [172], the width of the radio footprint in the shower plane [173] or the slope of the frequency spectrum in single radio antennas [101]. In addition to the Xmax reconstruction from radio data, the AERA measurement
can be combined with underground particle detectors, which measure the muonic component of the shower, to estimate the primary particle type [170].
4.4
Upgrade of the Observatory
The upgrade of the Auger Observatory, known as AugerPrime, will allow us to es- timate the primary mass of the highest-energy cosmic rays on a shower-by-shower basis [174]. The upgrade will search for light primaries at the highest energies, aim- ing to reach a sensitivity as small as 10% in the flux contribution of protons in the suppression region of the cosmic-ray energy spectrum [175]. Moreover, it will per- form composition-selected anisotropy studies as well as search for new phenomena including unexpected changes in hadronic interactions [174]. The upgrade consists of the installation of a new detector above each of the 1660 existing WCD. The de- tectors are plane plastic scintillators, named Surface Scintillator Detector (SSD), and they will be triggered by the WCD below it. The shower particles will be sampled with two detectors having different responses to muons and electromagnetic par- ticles, providing a complementary measurement of the shower particles. An SSD unit consists of a box of 3.8 m × 1.3 m containing two scintillator modules. The modules are composed of 24 scintillation bars produced at the Fermi National Ac- celerator Laboratory with dimensions of about 1.6 m length, 5 cm width and 1 cm thickness [175]. The scintillators are mounted on the top of the existing WCD (see Figure 4.14) and are read out by wavelength-shifting fibers which guide the light of the two modules to a PMT (model Hamamatsu R9420) [174].
FIGURE4.14: 3D view of an SSD mounted on a WCD. A double roof, with the upper layer being corrugated aluminum (here shown partially cut away for
clarity), is used to reduce the temperature variations [174].
Another important improvement of the Observatory is the upgrade of the electronics of the WCD that will process both WCD and SSD signals [175, 176]. This upgrade will increase the data quality due to a better timing accuracy and a faster sampling for Analog-to-Digital Converter (ADC) traces, as well as to improve the capabilities of the surface-detector calibration and monitoring. Since the Auger- Prime design aims to measure the properties of air showers at energies above 6×
1019eV at distances close to 250 m from the shower core, the WCD is equipped with an additional small PMT (1 inch Hamamatsu R8619 PMT) which is dedicated to the unsaturated measurement of large signals. The large-PMT signals are calibrated with background muons, while for the cross-calibration between the large and small PMTs either small shower events are used or else the existing LED flasher system which is adapted for brighter light pulses [176].
The Engineering Array of the upgrade has already been started since September 2016 with the deployment of the first twelve stations of AugerPrime. About the location of the Engineering Array, nine detectors are located in the ar- ray where the surface-detector separation is 1500 m and three detectors in the 750 m spaced array. They have been taking data since the beginning of October 2016 and have already permitted the reconstruction of more than 3000 cosmic-ray events [175].
The Auger upgrade promises high-quality future data from 2018 until 2024. The event statistics collected with the upgraded detectors will be comparable to the existing Auger data set, with the critical added advantage that every event will now have mass information. This will allow us to better understand the origin of the flux suppression, the prospects of light-particle astronomy and secondary particle fluxes, and the possibility of new particle physics at extreme energies.